Heretofore, it has been common practice to employ, for a membrane for brine electrolysis or for a membrane or a catalyst layer of a polymer electrolyte fuel cell, a polymer obtained by hydrolyzing a copolymer of a fluoromonomer represented by CF2═CF— (OCF2CFY)m—Op—(CF2)n—SO2F where Y is a fluorine atom or a trifluoromethyl group, n is an integer of from 1 to 12, m is an integer of from 0 to 3, p is 0 or 1 and m+p>0, with tetrafluoroethylene, or a polymer having a sulfonic acid group (the polymer having a sulfonic acid group will be referred to hereinafter as a “sulfonic acid polymer”) obtained by further converting the hydrolyzed copolymer to an acid form.
Since the above-mentioned sulfonic acid polymer has a softening temperature of near 80° C., an operation temperature of the fuel cell using the polymer is normally at most 80° C. However, in a case where a fuel gas for a fuel cell is hydrogen obtained by reforming an organic compound composed of carbon and hydrogen atoms or composed of carbon, hydrogen and oxygen atoms, such as methanol, natural gas or gasoline, carbon monoxide, if contained even in a trace amount, will poison an electrode catalyst, and it will be likely to result in a drop of output power of the fuel cell. There are, therefore, strong demands for increase of the operation temperature, in order to prevent the drop of the power. Furthermore, the increase of the operation temperature is also demanded in order to miniaturize a cooling apparatus for a fuel cell. However, the above-mentioned conventional polymer has the softening temperature too low to meet these demands.
Japanese Patent 2,675,548 offered a proposal on use of a sulfonic acid polymer having a short side chain and a high softening temperature as an electrolyte for a fuel cell. However, since such a sulfonic acid polymer is produced with difficulties and at high cost, it has not been produced in practice.
It is conceivable that if a polymer has a ring structure, a softening temperature will generally increase and it becomes possible to generate power at a temperature higher than the present level. However, the conventional technology for introducing an ionic group such as a sulfonic acid group into a polymer was copolymerization with a monomer having this ionic group and a polymerization moiety of vinyl ether, but it had a problem that the softening temperature of the polymer was not sufficiently increased.
On the other hand, there has been no conventional example of synthesis of a monomer having an ionic group, such as a sulfonic acid group, or its precursor group, a ring structure and a polymerization moiety with polymerization reactivity higher than that of a perfluorovinyl ether. JP-A-2001-522376 exemplifies monomers of (A) to (E) below, which are, however, hard to synthesize in fact, and describes neither a synthesis method nor a synthesis example. Furthermore, there is no description on a polymer having a repeating unit based on the monomer. In the formulae below, M′ is hydrogen, an alkali metal, an alkaline-earth metal, or the like, X is a fluorine atom, a chlorine atom or a trifluoromethyl group, n is 0 to 10, and Q is a hydrogen atom, a fluorine atom, a cyano group, an alkyl group, an SO2R(R is an alkyl group or the like), or the like.

U.S. Pat. No. 4,973,714 discloses a monomer represented by formula (F) below where X represents one of various functional groups such as F, Cl, —OC6F5, —CN, —COF, —COOR(R is —CH3, —C2H5 or —CH2CF3), —SO2F and —SO2Cl. However, it is difficult to synthesize a compound wherein X is —SO2F or —SO2Cl (Rf1 is a fluorine atom or a perfluoroalkyl group and Rf2 is a perfluoroalkenyl group which may contain an oxygen atom of an ether bond type), and the U.S. patent describes no synthesis example thereof.

JP-A-2002-260705 describes that a polymer electrolyte having a ring structure has a high softening temperature as a polymer and that increase of output can be achieved by applying it to an electrolyte contained in a cathode of a fuel cell. In this document, a perfluorovinyl ether monomer having a sulfonic acid group or a functional group convertible into a sulfonic acid group (which will be referred to hereinafter as a “sulfonic acid type functional group” in the present specification), but having no ring structure is copolymerized with a monomer having no sulfonic acid type functional group and having a ring structure or cyclopolymerizability, thereby obtaining a polymer having a ring structure and a sulfonic acid group. However, such a polymer fails to sufficiently increase a ratio of the ring structure to the whole polymer. In addition, it was difficult to produce a polymer with a high molecular weight if the above-mentioned perfluorovinyl ether monomer was used.